The present invention is in the field of imaging systems. More particularly, the present invention provides an autofocus system for an imaging system that incorporates a feedforward signal to increase the effective tracking bandwidth of the autofocus system.
In external drum imaging systems, a movable optical carriage is used to displace an image exposing or recording source in a slow scan direction while a cylindrical drum supporting recording material on an external surface thereof is rotated with respect to the image exposing source. The drum rotation causes the recording material to advance past the exposing source along a direction that is substantially perpendicular to the slow scan direction. The recording material is therefore advanced past the exposing source by the rotating drum in a fast scan direction.
An image exposing source may include an optical system for scanning one or more exposing or recording beams. Each recording beam may be separately modulated according to a digital information signal representing data corresponding to the image to be recorded.
The recording media to be imaged by an external drum imaging system is commonly supplied in discrete sheets and may comprise a plurality of plates, hereinafter collectively referred to as xe2x80x9cplatesxe2x80x9d or xe2x80x9cprinting plates.xe2x80x9d Each plate may comprise one or more layers supported by a support substrate, which for many printing plates is a plano-graphic aluminum sheet or a polyester substrate. Other layers may include one or more image recording (i.e., xe2x80x9cimageablexe2x80x9d) layers such as a photosensitive, radiation sensitive, or thermally sensitive layer, or other chemically or physically alterable layers. Printing plates are available in a wide variety of sizes, typically ranging, e.g., from 9xe2x80x3xc3x9712xe2x80x3, or smaller, to 58xe2x80x3xc3x9780xe2x80x3, or larger.
During high-resolution imaging, each recording beam is generally tightly focused to produce a small spot having a predefined spot size at the image plane (e.g., the imageable layer of the recording media supported on an external drum). Because of the smallness of the spot at the image plane, the spot profile changes very rapidly away from this plane. This causes a rapid change in the spot size and/or energy density, potentially causing a degradation of the quality of the recorded image with slight defocussing. The spot profile can be adversely affected by a large number of factors, such as variations in the finish of the drum surface (drum runout) and media distortion (e.g., thickness variations in the media). To compensate for these factors, autofocus systems are commonly incorporated into high-resolution imaging systems.
In external drum imaging systems in which the drum is rotated at a relatively low speed, currently available autofocus systems are capable of adequately maintaining the recording beam(s) in focus. In external drum imaging systems in which the drum is rotated at a high speed, and/or when using recording media with local distortion, however, an autofocus system with a high tracking bandwidth is required. Unfortunately, mechanical resonances within the imaging system impose a limit on the tracking bandwidth and, consequently, image quality suffers.
A need therefore exists for an autofocus system that is capable of providing a high effective tracking bandwidth required by high speed external drum imaging systems.
The present invention provides an autofocus apparatus and method with improved effective tracking bandwidth that utilizes a control loop with scan line to scan line feedforward of a motor command.
Generally, the present invention provides an apparatus, comprising:
an autofocus system for automatically focusing each of a plurality of scan lines produced by an imaging system on a supply of recording material, the autofocus system including a control loop with scan line to scan line feedforward of a motor command waveform.
The present invention additionally provides an autofocus system, comprising:
a focus detector for determining a focus error of a current scan line being imaged on a supply of recording media by an imaging system, and for generating a motor command waveform corresponding to the focus error;
an adder for adding the final motor command waveform of the previous imaged scan line to the motor command waveform of the current scan line to generate a final motor command waveform for the current scan line; and
a motor for controlling the imaging system to focus the current scan line on the recording media in response to the final motor command waveform of the current scan line.
The present invention also provides an autofocus method, comprising:
imaging a scan line on a supply of recording media;
determining a focus error of the scan line using positional information (e.g., final motor command information as a function of position) from a focus correction for a previously imaged scan line; and
focusing the scan line on the recording media based on the focus error.
The present invention further provides an imaging system, comprising:
a supply of recording media;
a media support surface for supporting the recording media during imaging;
a scanning system for imaging a plurality of scan lines on the recording media; and
an autofocus system, coupled to the scanning system, for automatically focusing each of the scan lines on the recording media, wherein the autofocus system includes a control loop with scan line to scan line feedforward of a motor command waveform.